In the polymerization of vinyl monomers (especially vinyl aromatic monomers), the resultant polymer can contain significant amounts of residual monomer. This frequently leads to undesirable properties such as high shrinkage in boiling water, poor heat deflection temperature, potential discoloration due to oxidation of the monomer, objectionable odor etc. The prior art describes many attempts to obtain vinyl aromatic polymers having a low residual monomer content (e.g. no more than 0.1% by weight). One means of reducing the residual monomer content is to include a co-initiator, typically in minor amount, that is, a finishing catalyst. Usually the finishing catalyst has a higher 10 hour half life temperature than the other initiator.
In one prior art method, (British Plastics, p. 26, January 1957), the polymer (polystyrene) is extruded through a devolatilizer where the residual monomer is removed at 225.degree. C. This method requires a large number of devolatilizers to handle large production volumes and the polymer must be heated to high temperatures which can lead to polymer degradation and poor physical properties.
U.S. Pat. No. 3,743,630 (issued July 3, 1973) to Wood describes the polymerization of styrene using ring substituted alkyl perbenzoates and branched chain alkyl perbenzoates. Compared with a t-butyl perbenzoate initiator, by using ring substituted alkyl perbenzoate (as a finishing catalyst) in combination with benzoyl peroxide, polystyrene (i.e. less than 0.1% by weight) with lower residual styrene can be obtained in less than 15 but more than 10 hours of polymerization time.
Canadian Pat. No. 915,850, issued Nov. 28, 1972 to Doak and Carrock describes the use of a minor amount of t-butyl peracetate or t-butylperoxy isopropyl carbonate (as finishing catalysts) and a major amount of benzoyl peroxide to obtain polystyrene which is substantially monomer free. The benzoyl peroxide is described as a low temperature organic peroxide initiator component. However, the polymerization time required is quite long (i.e. greater than 10 hours). The polymerization is conducted in suspension in two separate heating stages. The first stage of the polymerization is conducted at 75.degree. C.-100.degree. C. and the second stage at 105.degree. C.-145.degree. C. (for 1-5 hours).
British Pat. No. 1,330,896 to Blakemore teaches using certain high temperature peroxides, such as 3,5-dimethyl-3,5-di-t-butylperoxy-1,2-dioxolane, as co-initiators to produce polymers with low residual monomer concentration. However, Blakemore reports that the residual styrene content of the resultant polystyrene was greater than 0.1%. Further, the co-initiators described by Blakemore also have the disadvantage that the polymerization temperature has to be quite high. For example in the two stage procedure described by Blakemore, the final stage of the polymerization is conducted at 175.degree.-185.degree. for 2 hours.
Blakemore in British Pat. No. 1,329,859 describes the use of other co-initiators such as 1,1,4,4,7,7-hexamethyl-cyclo-4,7-diperoxynonane, which require excessively high polymerization temperatures.
Beresniewicz (U.S. Pat. No. 3,534,009) describes a method for reducing the residual vinyl acetate monomer content during the copolymerization of vinyl acetate and ethylene. After the main copolymerization the pressure is reduced from 100-3000 psi to atmospheric pressure and a source of free radicals (e.g. peroxide initiator) is added to the system. to reduce the residual vinyl acetate content to less than 0.35%.
Thompson and Jursich (Ref. U.S. Pat. No. 3,414,547) describe a polymerization process wherein a combination of a peroxide and an azo initiator is used to effectively reduce the residual monomer concentration. Most of the polymerization is done by the peroxide initiator and the azo initiator is the finishing catalyst. Catalysts other than the azo compounds are disclosed as not capable of reducing the residual monomer content below 2% by weight.
Bergmeister and Stoll (Ref. Ger. Offen. No. 2,229,569) describe a mixed catalyst system for preparing vinyl acetate copolymers. Using 2,2-bis(t-butylperoxy)butane in combination with another peroxide, such as benzoyl peroxide, produces a copolymer with a residual monomer concentration of 0.3% by weight as opposed to 2.1% by weight of vinyl acetate in the absence of 2,2-bis(t-butylperoxy)butane. A residual monomer concentration of 0.3% is not acceptable in most applications and especially in polymers which are used in contact with food.
The abstract of Japanese patent 74200/76 in Derwent Japanese Patents Report (Vol. 74, No. 21, page A + E-2, issued June 25, 1974) describes a process for the manufacture of copolymers of alpha-methylstyrene, acrylonitrile and styrene using azobiscyclohexanenitrile in combination with dicumyl peroxide and/or di-t-butyl peroxide as the initiators in a temperature range of 80.degree.-130.degree. C.
Guillet and Towne (U.S. Pat. Nos. 3,337,602 and 3,287,337) disclose novel peroxides useful as polymerization initiators, which have the general structure, ##STR3## where R is the same or different aliphatic, cycloaliphatic or aromatic hydrocarbyl or substituted hydrocarbyl, desirably containing 5 to 20 carbon atoms.
Other patents which contain teachings relevant to lowering residual monomer content in polymers are U.S. Pat. No. 3,784,532, to Fellmann and Uhang and Canadian Pat. No. 751,552 (which discloses the use of di-t-butyl-diperoxycarbonate as a polymerization initiator, but not as a finishing catalyst).